Spectrophotometer

ABSTRACT

A spectrophotometer, comprising a housing in which a measuring system is arranged, which housing has a measuring opening, via which light is passed to the measuring system. The measuring system comprises a grating monochromator, an autocollimator cooperating therewith, and detection means for the light originating from the grating monochromator. The grating monochromator and the autocollimator thereby form one whole.

RELATED APPLICATIONS

This application is a 35 U.S.C. §371 national phase application ofPCT/NL99/00778 (WO 00/45140), filed on Dec. 16, 1999, entitled“Spectrophotometer,” which claims priority to the NetherlandsApplication Number 1011146, filed Jan. 27, 1999.

The present invention relates to a spectrophotometer, comprising ahousing in which a measuring system is arranged, which housing has ameasuring opening, via which light is passed to the measuring system,and whereby the measuring system comprises a grating monochromator, anautocollimator cooperating therewith, and detection means for the lightoriginating from the grating monochromator. More in particular, theinvention relates to such a spectrophotometer, which also comprises anilluminator in a 45°/0° configuration with a light source formed by alamp and an illuminator optic, whereby after reflection light emitted bythe lamp is passed via a measuring opening into a housing.

A spectrophotometer as defined in the opening paragraph is known fromthe international patent application WO82/0163. For such a measuringsystem a spectrophotometer ISO standards have been fixed; in the mostrecent ISO standards for color measurement an optical band width of 10nm is recommended and a maximum value of 20 nm is prescribed. Moreover,the measuring system must be sensitive to collimated light to complywith these standards, whereby the rays entering the housing are allowedto deviate from the optical axis by up to 5°. The sensitivity to lightfrom other directions must be minimal.

As indicated, light refraction takes place by means of an opticalreflection grating. The detection often takes place by means of an arrayof photosensitive cells, in particular an integrated circuit with anarray of photodiodes. In addition, other methods of light refraction anddetection are used in practice. Known is, or instance, light refractionby means of a prism or a number of constant color filters or a linearlyvariable color filter. The detection may also occur with one singlemeasuring cell, whereby for measuring different wavelengths the opticalgrating or the color filter device is rotated, for instance with astepping motor with which the whole measuring range can be scannedwithin a few seconds.

Measuring techniques by means of grating monochromators are extensivelydescribed in E. G. Loewen, E. Popov; Diffraction gratings andapplications (Marcel Dekker, Inc., New York, 1997), in particular inChapter 12 thereof. Such techniques can be divided into two maincategories: constructions with plane reflection gratings andconstructions with concave reflection gratings. Nearly all theconstructions with plane reflection gratings make use of one or moreconcave mirrors for collimating the light. A known exception is theso-called Littrow arrangement, which, however, is designated in theabove literature as out-of-date (page 444). The concave reflectiongratings are relatively expensive, but offer various advantages, inparticular because of their applicability in the UV range and because oftheir simple construction. As a special advantage it holds that thefunctions of dispersion and autocollimation are combined therein; otheroptical components are not required therefor, which also preventslining-up and stability problems.

For plane reflection gratings the Monk-Gillison arrangement isdesignated as the simplest and cheapest system (see the aboveliterature, paragraph 12.5). In this system only two components are usedfor dispersion and collimation, namely a concave mirror and a reflectiongrating.

The object of the invention is to provide the spectrophotometer with ameasuring system which means a further simplification with respect tothe Monk-Gillison arrangement, which is additionally inexpensive andalso complies with the above ISO standards.

According to the invention the spectrophotometer as defined in thepreamble is characterized in that the grating monochromator and theautocollimator form a grating lens which, as a single, physicalstructure, has on one side convex collimator lens structure and on theother side an externally mirrored grating structure, the plane of thegrating structure being inclined at a relative small angle to theoptical axis of the measuring system and/or to the optical axis of thecollimator lens. The edges of the grating lens are blackened toextinguish undesired reflections.

From U.S. Pat. No. 4,838,645 a reflecting diffraction grating is knownin which a grating monochromator and an autocollimator which form onewhole optical system, but not a single physical structure.

In the simplest form the grating lens is made of an optically brightplastic, preferably acrylate glass (PMMA). By using inexpensivemanufacturing methods such as injection molding or pressing, the costprice of such a lens can be low. It is also possible to co-formfastening edges which facilitate the mounting, Also, the mirroring ofthe grating lens with aluminum evaporated under vacuum can be realizedin an inexpensive manner in a mass production.

The grating structure can be made in a mold for the lens in the form ofa so-called ruled grating. The original master grating can be notchedwith a diamond chisel in an optically pure plane substrate by means of aso-called ruling engine. Ruled gratings have a higher efficiency thanholographically made gratings. By making the proper selection of theblaze angle, the efficiency can be optimized for the desired measuringrange, in this case the visible light spectrum. This is an advantageover concave gratings, with which it is difficult to obtain a comparableefficiency.

In a preferred embodiment the measuring system comprises a Littrowarrangement whereby light enters the housing in a first direction(Y-direction) and falls therein on the grating lens via an entrance slitand a reflecting element in a direction substantially perpendicularthereto, the negative Z-direction, while reflected light from thegrating lens substantially falls on the detection means in the positiveZ-direction. The measuring system thereby depicts the slit on thedetection means.

To keep the rays falling on and reflected by the grating lens separated,the angle therebetween in the YZ-plane is in the order of 15 to 20°. Itwill be bright that this angle value is only motivated by practicalconsiderations. The optical axis of the lens curvature in theX-direction, perpendicular to the YZ-plane, falls into the YZ-plane,namely in the Z-direction. The plane of the grating is inclined at anangle in the order of 6° to the XY-plane.

The detection means are formed by an array of photosensitive cells, thedimensions of which are in the order of 0.2×0.2 mm or less, while,furthermore, a cylindrical lens is present to converge the light fromthe grating lens on the photocells. The entrance slit extending in theZ-direction often has a size of about 2 mm, while the width thereofextending in the X-direction is about 0.2 mm, which corresponds with aband width of about 10 nm, so that, when the dimensions of thephotocells is of the same order as the size of the projected entranceslit, an array length of 6 to 9 mm is necessary to enable depicting ofthe visible spectrum. Arrays of narrow long photocells in the size ofthe above slit are certainly manufactured, but they are considerablymore expensive than the more conventional arrays, the cells of whichhave a smaller and substantially square cross-section, often in theorder of 0.1×0.1 mm. For reasons of cost price, it is favorable to usethese arrays, but the light-sensitivity of such small photocells isoften lower. To compensate this drawback, the array in the deviceaccording to the invention is provided with a cylindrical lens whichconverges the about 2 mm high picture on the small photocells, whichcauses the local light intensity to be increased proportionally. Becauseof the small dimensions of the array the focal distance of the gratinglens may also be small and thus the whole optical system. The wholehousing of the measuring system can therefore be kept within thedimensions of 3×3×5 cm.

Scattered light and radiation of higher orders must be reduced orstopped in the conventional manner by providing stop filters in theoptical path. The position of such stop filters, which have to act on apart of the useful spectrum, is just before at least part of thephotocells. These filters can, for instance, be cemented to thecylindrical lens, thereby preventing additional surface reflections. Aninfrared stop filter may be arranged at the entrance of the system toprevent infrared scattered light to which silicon photodiodes are verysensitive. This filter is of such quality that the whole desiredspectrum is sufficiently passed. Moreover, at a number of places in thehousing of the measuring system plates are arranged which only leave anopening for the desired light bundle and thus form chambers in whichscattered light extinguishes. This is in particular important toseparate white light reflections.

In the simplest embodiment of the grating lens this lens is symmetricalwith respect to rotation, the axis being directed between the virtualposition of the slit and the middle of the array. A part of the lightfalling on the grating lens, at least 4%, will not pass through the lenspart thereof to be mirrored back after diffraction, but already bemirrored back on the surface as white light; such reflections are calledFresnel reflections. Because a part of the white light is mirrored tothe array, measuring errors arise. These are even intensified becausethe energy of the white light is much higher than that of themonochromatic light which is nominally measured by the photocells. In apreferred embodiment of the grating lens this is prevented by an“off-axis” design. The lens is, as it were, placed obliquely backwards,so that this scattered light reflects back at such an angle that it doesnot reach the array. This oblique placement causes considerable changesin the optical action of the lens. Because of the asymmetry, more imageformation (coma) appears which adversely affects the optical band widthof the measurement. Also, the angle of the grating with respect to theY-axis in the XY-plane must be changed.

In a further improved embodiment the curvature of the lens in theY-direction is made less strong than in the X-direction. The lens thusobtains a toric shape. It has been found that at a proper selection ofthe curvature in the Y-direction the optical deformation is stronglydecreased, while the lens still sufficiently converges in theY-direction. The cylindrical lens for the array needs to be onlyslightly magnified.

In yet another embodiment the lens is cylindrically shaped with only acurvature in the X-direction. In that case the image formation canbecome even smaller in the inclined position required to prevent Fresnelreflections. The picture of the entrance slit, however, thereby becomeseven larger in the Y-direction, which may lead to an impractically largecylindrical lens or to a loss of sensitivity.

Furthermore, it proves to be also possible to inhibit Fresnelreflections when the collimator lens structure, that is to say thegrating lens, is designed as one having a symmetric or slightlyasymmetric toric shape, with a damping element being arranged in or nearthe middle.

The monochromator is often defined from the entrance slit. Theadmissible opening angle determines the light intensity of the system.Conventional is a numerical aperture of slightly larger than 0.1, whichis half an opening angle of 6 to 8°. At a larger lens opening too muchdeformation occurs. In the grating lens according to the invention theoptical deformation at a large aperture increases even more stronglythan in the known multi-element systems. However, for use as acolorimeter this is no drawback or limitation because ISO standards forreflection measurement prescribe a small aperture. The half openingangle may be at most 5°.

In surface color measurement it is conventional to measure anapproximately circular surface having a diameter of a few mm and nothaving a relatively long narrow slit. The testing targets are oftensmall square color areas. In this optical system a good adaptation to anapproximately round measuring opening can be obtained by making use of alens which converges collimated light from the measuring opening in theX-direction on the entrance slit, while the lens contrarily diverges inthe Y-direction through the longitudinal direction of the slit until atthe total height of the grating lens. Therefore, in a concreteembodiment the measuring opening in the housing is substantiallycircular and between this measuring opening and the entrance slit asaddle-shaped lens is present which converges collimated light from themeasuring opening in the X-direction, perpendicular to the YZ-plane, onthe entrance slit, while this lens diverges the light reflected by thereflecting element in the Y-direction. Such a lens shape may also beinexpensively produced in plastic, for instance by an injection moldingtechnique. Preferably, saddle-shaped lens is, in particular on the planeside, provided with an infrared stop filter, so that no infrared energyenters through the entrance slit into the housing of the measuringsystem.

Because the elements of the array of photosensitive elements aresensitive to various wavelength intervals in different ways, one or moreblocking filters adjusted to different wavelength intervals may bearranged before the array of photosensitive elements. Very accuratefilters are obtained in the form of an integrally formed blocking filtercombination of filters secured against each other, the transition areaextending obliquely in two directions X and Y or the transition areasextending obliquely parallel to each other in two directions X and Y.Here a transition area may be inclined in the X direction at an angle βof 10 to 70°, preferably of 10 to 40°, in particular approximately 20°,and in the Y direction at an angle α of 10 to 70°, preferably 30 to 60°,in particular 45°.

The spectrometer according to the invention is an inexpensive, handy andreliable instrument for measuring surface colors of printing matter,paint, plastics, textiles, foodstuffs, etc. In particular withoutilluminator the spectrophotometer may serve for measuring light sources,for instance for displays, for inspection of theater and studiolighting, office and street lighting. This spectrophotometer is inparticular suitable for connection to a computer in which theinformation of the detection means can be processed.

The invention will now be explained in more detail with reference to theaccompanying drawings, in which:

FIG. 1 shows a schematic structure of a spectrophotometer with ameasuring system according to a Littrow arrangement;

FIG. 2 shows a measuring system according to the invention, viewed inthe YZ-plane;

FIG. 3 shows the measuring system of FIG. 2, viewed in the XZ-plane;

FIG. 4 shows the measuring system of FIGS. 2 and 3, viewed in theXY-plane;

FIG. 5 is a perspective view of the measuring system of FIGS. 2 and 3,which the operation of the saddle-shaped lens in the XY-plane is alsoshown;

FIG. 6 is a more enlarged view of the measuring system, viewed in theYZ-plane, in which the operation of the saddle-shaped lens in theYZ-plane is also shown;

FIG. 7 is a similar front view of the measuring system as in FIG. 6, inwhich, however, Fresnel reflections are shown;

FIG. 8 shows an alternative way of minimizing the effects of Fresnelreflections on the detection;

FIG. 9 shows a detector arranged in the measuring system, provided withblocking filters;

FIG. 10 shows a blocking filter combination, the two filters of whichare secured straight against each other;

FIG. 11 shows a blocking filter combination, the two filters of whichare arranged obliquely against each other;

FIG. 12 shows a three-fold blocking filter combination, the middlefilter of which has the shape of a parallelepiped; and

FIG. 13 shows a four-fold blocking filter combination.

The spectrophotometer shown in FIG. 1 comprises a housing 1, whichencloses a measuring system 2 according to a Littrow arrangement with areflecting element 3, a collimator lens 4, a grating monochromator 5,and a detector 6. As such a measuring system is known from E. G. Loewen,E. Popov; Diffraction gratings and applications (Marcel Dekker, Inc.,New York, 1997), see in particular Section 12.5, the operation of thissystem needs no further discussion. The embodiment of thespectrophotometer described herein is intended to carry out reflectionmeasurements. In this embodiment, a substantially parallel bundle 9,reflected by a specimen S, is supplied to the measuring system 2 via anentrance opening 7 and a lens 8.

The spectrophotometer comprises an illuminator 10 with an illuminatoroptic 11 and a lamp 12. The illuminator optic is formed by lightconductors 13 which start at the lamp 12 and end in the form of a ringwith a conoid-shaped radiating side. Such an illuminator optic is, forinstance, known from U.S. Pat. No. 4,464,054. In this optic, the surfaceof the specimen 7 to be exposed is radiated at an angle of 45°, inaccordance with the valid ISO standards, while a tolerance not exceeding±5° appears. Of the light reflected and scattered by the specimensurface, a light bundle 9 is supplied via the opening with the lens 8 tothe measuring system 2, perpendicularly to this surface. The lens 8 isof such design that the tolerance in the radiation direction of thebundle 9 also does not exceed ±5°. Since the invention relates to themeasuring system and not to the illuminator, the operation thereof needsno further discussion. It will be clear that all kinds of possible knownilluminators which comply with the valid ISO standards can be used here.

An exemplary embodiment of a measuring system for a spectrophotometer isshown in FIGS. 2-7. These figures also show an X,Y,Z-coordinate system,by means of which the measuring system will be described. The measuringsystem in these figures comprises a plane grating monochromator 14, anautocollimator 15 cooperating therewith, and detection means 16. Thegrating monochromator 14 and the autocollimator 15 form one whole, whichwill further be designated as grating lens 17. This grating lens has onone side a convex collimator lens structure and on the other side anexternally mirrored grating structure. The convex collimator lensstructure has an asymmetric toric shape. The grating lens is made of anoptically bright plastic, in particular of acrylate glass, the edgesbeing finished in black. The plane of the grating structure is inclinedat an angle of approximately 6° to the XY-plane of the measuring system(see FIG. 3). The grating structure of the grating lens 17 is made in amold in the form of a so-called ruled grating, the original mastergrating being notched with a chisel in an optically pure plane substrateby means of a so-called ruling engine.

The light bundle 9 passes through a round measuring opening 18 in theY-direction (see FIG. 5), this is the entrance opening 7 in FIG. 1, intothe housing 1. Via a saddle-shaped lens 19, an entrance slit 20 (seeFIG. 5), and a reflecting element 21, the light rays move approximatelyin the negative Z-direction to the grating lens 17. The light decomposedby means of the grating lens 17 is then supplied, in the positiveZ-direction and by means of a cylindrical lens 22, to an approximately 6to 9 mm long array of photocells 23, the cell dimensions of which aresmaller than 0.2×0.2 mm, in the present embodiment 0.1×0.1 mm. Thecylindrical lens 22 and the array of photocells form the detection means16.

To keep the incoming and outgoing bundles separated from the gratinglens, the mirroring element 21 is placed at an angle to the XZ-planeslightly exceeding 45°, which results in an angle between the incomingand outgoing bundles from the grating lens of approximately 15 to 20°(see FIG. 2). By tilting the grating lens 17 in the YZ-plane slightlybackwards, as can be seen in FIG. 2, Fresnel reflections, as shown inFIG. 7, remain out of range of the detection means, while, furthermore,Fresnel reflections appearing at the cylindrical lens 22 owing to thestrong curvature also exert substantially no disturbing effect.

By means of the saddle-shaped lens 19, the light entering the housing 1through the round opening 18 is focused in the XY-plane on the entranceslit 20. The converging properties of the lens 19 in the XY-plane arethen such that substantially only the (collimated) light entering at 0°falls on the grating lens. The saddle-shaped lens 19 further hasdiverging properties in the YZ-plane (see FIG. 6), which are such thatthe (collimated) light entering substantially at 0° mainly covers theentire height of the grating lens. The light bundle leaving the gratingis then focused by the cylindrical lens 22 on the array of photocells23.

The saddle-shaped lens 19 is provided on the plane side with an infraredfilter. Furthermore, plates, light-absorbing elements, stop filters, andthe like, may be arranged in the housing 1 to eliminate the effects ofundesired light diffusion as much as possible. Thus, the figuresschematically show a plate 24 with a passage opening for the bundlereflected by the mirroring element (see FIG. 5). Furthermore, as shownin FIG. 3, plates 25 and 26 may be present to enable suppression andabsorption of the effects of Fresnel reflections and other appearingscattered light. Of these plates, the position in the Z-direction ispreferably equal to that of the plate 24 in FIG. 2.

FIG. 8 shows an alternative arrangement for preventing Fresnelreflections on the lens surface of the grating lens 17. To this end, asmall damping element 27 is arranged in the middle of the lens, in theform of a black small plane or a small pyramid of black plastic. Throughthe spherical shape of the lens surface, which diverges reflections butconverges transmissions, only a relatively small element is required toprevent reflections from finding their way to the array of photocells23. As appears from FIG. 8, a part of the desired light is stopped aswell. It turns out, however, that this is less than 15% of the totallight amount. The advantages of this construction are that it is notnecessary for the grating lens 17 to be asymmetric, so that the imageformation through coma is smaller, and that a symmetric lens can bemanufactured in a less expensive way with the required accuracy. In aninjection molded product, the asymmetry leads to more deformationthrough material shrinkage during the solidification phase. It is notedthat the grating lens does retain its toric character.

It is possible, however, to design the grating lens 17 to be slightlyasymmetric in the Y-direction, while placing the damping element alittle outside the middle, in particular in a slightly lower position.

As shown in FIG. 9, blocking filters 28 can be placed before the arrayof photocells 23. These filters are used to block higher orderreflections, in particular second order reflections of the grating lens17. By placing filters (long pass filters) before the array cells forthe range of 6,000 to 8,000 nm, the coincident second order radiation inthe range of 300 to 400 nm is prevented from reaching these cells. Also,filters (short pass filters and/or band pass filters) can be used in therange of 300 to 5,000 nm to ensure that red scattered light is retained.Since the employed photodiodes are more sensitive to red light than toblue light, it is conducive to the measuring accuracy in the blue partof the spectrum to retain even the least red scattered light with ablocking filter.

The blocking filters 28 are preferably placed just before the photocellarray 23. In particular, they are cemented to the cylindrical lens 22and the array 23 with an optical resin. The filters have a physicalthickness in the Z-direction in the order of 1 mm.

In the simplest form, two filters, a blue-green filter F1, mostly ofglass, and a yellow, orange or red filter F2, of glass or plastic, areplaced straight against each other, as shown in FIG. 10. Smalldifferences in the refractive index between the filter and the opticallybright resin with which they have been cemented, will cause reflectionsto appear on the side of the filters at the transition O of the twofilters for light rays passing through the glass surface at a smallangle with respect to the normal. Often, the refractive index, both ofthe filter and of the cement, is in the order of approximately 1.54 to1.57, but never exactly equal. A difference in the refractive index inthe order of approximately 1% is often unavoidable. By means ofSnellius' refraction law, it can be determined that, when the angle ofincidence comes to exceed approximately 82° to the normal, totalreflection appears. This means that light entering at an angle less than8° with respect to the z-axis cannot pass through the boundary face O,but is reflected thereon. Since the light from the grating lens 17passes through the filters exactly at such small angles with respect tothe z-axis, optical errors reducing the measuring accuracy will developin the surroundings of the transition O. As appears from FIG. 10, a partof the light rays from directions between AM and BM, which are directedto the photocell M of the array 23, will find its way to the photocell Nthrough reflection on the boundary face O. This causes light of awavelength pertaining to the cell M to be measured as if it had thewavelength pertaining to cell N, which results in an enlargement anddeformation of the band width of the spectrometer in the surroundings ofthe boundary face O of the filters F1 and F2. Another drawback of theconstruction shown in FIG. 10 is that the blocking properties directlyto the left and right of the boundary face O are so much different thatin many cases a small, but disturbing leap in the spectrophotometerresponse will occur at the location of the transition as a result of adifference in scattered light contribution. To reduce these errors, itis known to make use of merging filters, as shown in FIG. 11. Thiscauses the effect of the transition O to be spread over a longer range,so that the errors are spread as well and thereby masked. The reflectionerrors, however, are not removed therewith, and especially in aspectrophotometer of the type in which the thickness of the filter glass(about 1 mm) is large with respect to the linear dispersion which is inthe order of 0.02 mm per nm, such a transition causes a significant lossof quality.

FIG. 12 shows an assembly of blocking filters F1, F2, and F3 accordingto the invention, adjusted to different wavelength intervals, with whichthe above errors can be considerably limited. The middle filter F2 hasthe shape of a parallelepiped with inclined angles α and β. The twotransition faces are mutually parallel, α being the angle at which thetransition faces extend in the Z-direction and β being the angle atwhich the transition faces extend in the X-direction. The outer filtersF1 and F3 are rectangular on the outer end, while they connect to theslanting sides of the filter F2 on the inner side. Although threefilters are used here, it is also possible to use two filters, forinstance by omitting the filter F2 in FIG. 12. Also, the number offilters can be increased by inserting several parallelepiped-shapedfilters. In a preferred embodiment, the angle α is approximately 45° andthe angle β about 20°. Of course, other angles can be chosen as well,which angles will particularly range between 10 and 70°, α rangingpreferably between 30 and 60° and β preferably between 10 and 40°. Thecomplicated filter shapes can be made of plastic by means of, forinstance, injection molds. Where glass filters are used, the shape mayoptionally be simplified to reduce the cost of processing by selecting ashape rectangular in one direction and causing the ends to protrude, asshown in FIG. 11. In this figure, F1 is a glass filter, and F2, F3 andF4 are plastic filters. Of course, even more glass filters can thus bearranged in combination. It is also possible to completely or partlysuperpose two or more layers of such filters. Combinations of blockingfilters can be formed therewith in the light path, so that the scatteredlight suppression can be improved even further. The blocking filteraccording to the invention, as described herein, can be used not only inthe spectrophotometer described herein, but in all possible types ofspectrophotometers.

The invention is not limited to the exemplary embodiment described withreference to the drawings, but comprises all kinds of modificationsthereof, of course as far as falling within the scope of protection ofthe appended claims. In particular, it is pointed out that theilluminator is quite optional and is only present for reflectionmeasurements. Without an illuminator, the spectrophotometer can also beused for, for instance, emission and transmission measurements.

What is claimed is:
 1. A spectrophotometer, comprising a housing inwhich a measuring system is arranged, which housing has a measuringopening, via which light is passed to the measuring system, and wherebythe measuring system comprises a grating monochromator, anautocollimator cooperating therewith, and detection means for the lightoriginating from the grating monochromator, characterized in that thegrating monochromator and the autocollimator form a grating lens which,as a single, physical structure, has on one side convex collimator lensstructure and on the other side an externally mirrored gratingstructure, the plane of the grating structure being inclined at arelative small angle to the optical axis of the measuring system and/orto the optical axis of the collimator lens.
 2. The spectrophotometeraccording to claim 1, characterized in that the edges of the gratinglens are black.
 3. The spectrophotometer according to claim 1,characterized in that the grating lens is made of an optically brightplastic.
 4. The spectrophotometer of claim 1, characterized in thatgrating structure of the grating lens is made in a mold for the lens inthe form of a ruled grating, whereby the original master grating isnotched with a diamond chisel in an optically pure plane substrate bymeans of a ruling engine.
 5. The spectrophotometer of claim 1,characterized in that the measuring system comprises an arrangementwhereby light enters the housing in a first direction (Y-direction) andfalls therein on the grating lens via an entrance slit and a reflectingelement in a direction substantially perpendicular thereto, the negativeZ-direction, while reflected light from the grating lens substantiallyfalls on the detection means in the positive Z-direction.
 6. Thespectrophotometer according to claim 5, characterized in that the anglebetween the incoming and outgoing light from the grating lens is locatedin the YZ-plane in the order of 15 to 20°.
 7. The spectrophotometeraccording to claim 5, characterized in that the optical axis of thegrating lens is inclined in the YZ-plane at a relatively small angle tothe optical axis between the grating lens and the detection means. 8.The spectrophotometer according to claim 1, characterized in that thedetection means are formed by an array of photosensitive elements, thedimensions of which are in the order of 0.2×0.2 mm or less, while,furthermore, a cylindrical lens is present to converge the light fromthe grating lens on the photosensitive elements.
 9. Thespectrophotometer according to claim 1, characterized in that thedimensions of the housing of the measuring system are less than 3×3×5cm.
 10. The spectrophotometer according to claim 1, characterized inthat the collimator lens structure is one having an asymmetric shape.11. The spectrophotometer according to claim 1, characterized in thatthe collimator lens structure is one having an asymmetric toric shape.12. The spectrophotometer according to claim 1, characterized in thatthe collimator lens structure is one having a symmetric or slightlyasymmetric toric shape, with a damping element being arranged in or nearthe middle.
 13. The spectrophotometer according to claim 5,characterized in that the measuring opening in the housing issubstantially circular, between which measuring opening and the entranceslit a saddle-shaped lens is present which converges collimated lightfrom the measuring opening in the X-direction, perpendicular to theYZ-plane, on the entrance slit, while this lens diverges the lightreflected by the reflecting element in the Y-direction.
 14. Thespectrophotometer according to claim 13, characterized in that thesaddle-shaped lens is provided with an infrared stop filter.
 15. Thespectrophotometer according to claim 8, characterized in that before thearray of photosensitive elements one or more blocking filters adjustedto different wavelength intervals are arranged.
 16. Thespectrophotometer according to claim 15, characterized in that theblocking filters are formed by filters secured against each other, thetransition area extending obliquely in two directions X and Y or thetransition areas extending obliquely parallel to each other in twodirections X and Y.
 17. The spectrophotometer according to claim 16,characterized in that a transition area is inclined in the X directionat an angle α of 10 to 70°, and in the Y direction at an angle α of 10to
 70. 18. The spectrophotometer according to claim 3, wherein theoptically bright plastic is acrylate.
 19. The spectrophotometeraccording to claim 16, characterized in that a transition area isinclined in the X direction at an angle α of 10 to 40° and in the Ydirection at an angle α of 30 to 60°.
 20. The spectrophotometeraccording to claim 16, characterized in that a transition area isinclined in the X direction at an angle α of 20° and in the Y directionat an angle α of 45°.